1. Field of the Invention
The present invention relates to a liquid crystal display device (hereinafter referred to as xe2x80x9cLCDxe2x80x9d), and more particularly to a displaying substrate formed with a pad portion capable of enhancing contact characteristics with a circuit substrate for applying electric signals to the displaying substrate, and an LCD having the same.
2. Description of the Related Art
Information-processing appliances have been rapidly developed to have a variety of shapes and functions and much faster information processing speed. Information in the form of an electrical signal processed in such an information-processing appliance requires a displaying device serving as an interface.
Compared with a CRT-type display device, a flat type display device has various advantages such as lighter weight and smaller size. Moreover, the flat type display device is able to realize full colorization and high resolution. The LCD is one of currently available flat-type display devices, which is provided with a liquid crystal display panel including two sheets of substrates formed with electrodes and a liquid crystal layer injected between the two substrates. Images are displayed in such a manner that the quantity of light transmitted is adjusted by applying a voltage to the electrodes of the liquid crystal display panel to rearrange the liquid crystal molecules of the liquid crystal layer.
Generally, in order to precisely display the detailed images on the liquid crystal display panel of the LCD, an image data generated from an external information processing apparatus should be converted into driving signals which are suitable for driving the liquid crystal display panel. The driving signals are applied to the liquid crystal display panel at proper timing.
For embodying such an operation, the LCD requires a process of signal processing upon the image data on a driving printed circuit substrate prior to being applied to the liquid crystal display panel.
In order to allow the LCD to carry out the full-color display, signal lines including gate lines and data lines are densely formed within a certain area of the TFT substrate. Also, a pad formed to be wider than the area occupied by the signal lines is connected to one end of the signal lines.
A medium for transmitting signals are required so as to apply the driving signals generated from the driving printed circuit substrate to the densely formed signal lines at an accurate timing. Chip On Glass (COG), Chip On Film (COF), Flexible Printed Circuit film (FPC) or Tape Carrier Package (TCP) are mainly employed as the medium. One end of the medium is connected to the driving printed circuit substrate while the other end thereof is electrically connected to the pad to be firmly fixed.
An interval between the pad and adjacent pad is too narrow as described above to utilize a method such as a fine welding typically employed for connecting the pad with the medium. For this reason, the pad is electrically and mutually connected with the medium by interposing an Anisotropic Conductive Film (ACF).
The ACF includes an adhesive tape and conductive particles regularly arranged in the adhesive tape. The conductive particle is smaller than several micrometers in diameter. That is, the conductive layer having an adhesive material and conductive particles and a passivation layer form the ACF. The conductive particles serve for applying the signal transmitted via the medium to the liquid crystal display panel via the pad. The conductive particles have one directional orientation in transmitting the signal. In other words, the signal from the medium can be applied to the liquid crystal display panel; on the contrary the signal from the liquid crystal display panel cannot be transmitted to the medium.
Additionally, the ACF is thermally compressed under the state of being interposed between the liquid crystal display panel and medium to be adhesively bonded therewith. The pad of the liquid crystal display panel and the medium are bonded with each other by means of the adhesive material, and further they are firmly attached to each other by a restoring force originated from an elastic deformation of the conductive particles.
FIG. 1 is a plan view for explaining a structure of a conventional pad, and FIGS. 2A to 2D are sectional views showing the manufacturing process of the pad shown in FIG. 1. FIGS. 1 to 2D show a gate pad structure which is extended from a gate line of an LCD. TCP is used as a medium. Referring to FIGS. 1 and 2A, a metal such as aluminum (Al) or chrome (Cr) is deposited on a substrate 60 generally formed of an insulating material, and is patterned to form a gate pad 10. Then, as shown in FIG. 2B, a silicon nitride layer is deposited on the entire surface of the substrate 60 formed with the gate pad 10 thereon via a plasma chemical vapor deposition (LPCVD) method, thereby forming a gate insulating layer 20.
As shown in FIG. 2C, an organic resist layer is coated on the entire surface of the gate pad 10 and a peripheral region thereof to form an organic insulating layer 30. In order to form an opening 11 in the gate insulating layer 20 to expose a portion of the gate pad 10, a mask 31 is placed over the organic insulating layer 30. Thereafter, the opening 11 for exposing the gate pad 10 is formed in the organic insulating layer 30 by an exposure and development process. The gate insulating layer 20 underlying the organic insulating layer 30 is also removed together to form the opening 11 that partially exposes the gate pad 10.
Then, as shown in FIG. 2D, a conductive layer 40 is formed along an inner surface of the opening 11 and organic insulating layer 30 at the periphery of the opening 11. The conductive layer 40 includes a metallic material such as aluminum or a transparent conductive material such as Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO).
Referring to FIG. 3, an anisotropic conductive film 70 includes conductive particles 71 and an adhesive material 72 is placed over the conductive layer 40 formed on the gate pad 10, and aligned with an output port 81 of a gate-side TCP 80. Then, a compressing operation is performed, to make an electrical connection between the conductive layer 40 and the output port 81 of the gate-side TCP 80 by means of the conductive particles 71.
The conductive particle 71 include a first conductive particle 71a, a seconds conductive particle 71b. The first conductive particle 71a is placed in the opening 11 and the seconds conductive particle 71b is placed on the surrounding portions of the opening 11. At this time, the difference in thickness of the second conductive particle 71b and the orgarnic insulating layer 30 is not large enough to transfer a sufficient compressing force upon the second conductive particle 71b when compressing the second conductive particle 71b onto the gate pad 10. Therefore, the height difference between the organic insulating layer 30 and the gate pad 10 induces a problem that the gate-side TCP 80 is poorly bonded to the gate pad 10 or detached therefrom even after being adhered thereto.
Furthermore, if the gate-side TCP 80 is mistakenly bonded onto the gate pad 10, the gate-side TCP 80 should be detached from the gate pad 10, and correctly bonded again thereto. For re-attachment, the substrate is wiped by means of a cotton swab and so on for eliminating foreign materials remaining on the gate pad 10. The organic insulating layer 30 is, liable to be detached from the gate insulating layer 20 because of weak adhesive strength between them and the step between the organic insulating layer 30 and the gate pad 10. As a result, the conductive layer 40 formed on the organic insulating layer 30 is detached together with the organic insulating layer 30 and placed between adjacent pads, thereby causing a short between the pads.
However, the organic insulating layer 30 functions to prevent an electrical short between adjacent gate pads. For this reason, if the organic insulating layer 30 is removed at and around the gate pad 10, an electrical short may occur between adjacent gate pads, which lowers reliability of the resultant product. Therefore, the organic insulating layer 30 should not be thoroughly removed from the portion where the gate pad 10 is formed.
The present invention provides a displaying substrate capable of enhancing connection characteristics between a pad portion and a circuit substrate, and reducing a driving failure.
Also, the present invention provides a liquid crystal display device having a displaying substrate capable of enhancing connection characteristics between a pad portion and a circuit substrate, and reducing a driving failure.
In one aspect, the displaying substrate according to the present invention includes at least one pad portion for receiving an electrical signal from a circuit substrate. The pad portion has a pad metal layer which is formed on the displaying substrate and has a pad area. A passivation layer covers the pad metal layer and has a plurality of via holes to partially expose the pad area. Also, a conductive layer is formed over the passivation layer corresponding to the pad area and is electrically connected to the pad metal layer through the via holes.
Each of output ports of the circuit substrate is electrically connected to the conductive layer of the corresponding pad portion by an anisotropic conductive film interposed therebetween.
A width of the via hole is smaller than a diameter of a conductive particle of the anisotropic conductive film.
A deformation ratio of the conductive particle is about 20xcx9c60%. The width of the via hole is smaller than 0.8 times of the diameter of the conductive particle.
A depth of the via hole is smaller than a diameter of the conductive particle. The depth of the via hole can be 0.8 to 0.4 times of the diameter of the conductive particle.
The via holes have various shapes such as rectangles, squares, circles or ellipses when viewed from an upper side of the displaying substrate.
The displaying substrate may be used for one of LCD, PDP, FED and EL.
In another aspect, a liquid crystal display device according to the present invention includes a liquid crystal display panel which has a pad portion and a plurality of pixel portions and a circuit substrate that has output ports electrically connected to the pad portion of the liquid crystal display panel for applying an electrical signal to the liquid crystal display panel.
The pad portion of the liquid crystal display panel has a pad metal layer that is extended from one end portion of a plurality of signal lines connected to the pixel portions and has a pad area. A passivation layer covers the pad metal layer and includes a plurality of via holes to partially expose the pad area, and a conductive layer is formed on the passivation layer corresponding to the pad area. The conductive layer is electrically connected to the pad metal layer via the via holes.
Each of the output ports of the circuit substrate is electrically connected to the corresponding conductive layer of the pad portion by interposing an anisotropic conductive film.
According to the displaying substrate and liquid crystal display device as described above, the pad portion has the via holes for exposing the pad metal layer. A width of the via hole is formed to be smaller than a diameter of the conductive particle of the anisotropic conductive film. Where the width of the via hole is larger than the diameter of the conductive particle, the depth of the via hole is formed to be smaller than the diameter of the conductive particle.
Consequently, the driving failure which may be generated from the pad portion can be prevented, and the deformation ratio of the conductive particles is about 20xcx9c60%. Thus, the connection force between the pad portion and circuit substrate can be enhanced.